The present invention relates generally to a lance for blow-refinement in a converter such as a Bessemer converter. More specifically, the invention relates to a lance having an auxiliary nozzle which can improve the thermal efficiency of secondary combustion in a converter.
As is well known, a lance for blow-refinement installed in the converter is directed to a molten metal bath for injecting a high-pressure, high-velocity jet of oxygen to cause strong churning and rapid reaction near the molten metal bath surface. High-purity, high-energy gaseous oxygen injected toward the molten metal bath surface causes a gas-metal reaction, specifically carbon reduction. At the same time, the oxygen flow causes a slag-metal reaction, such as slagging of lime, and scavenging of phosphorus. When the proportion of pig iron in the source material is relatively high, specifically approximately 95%, the carbon content in the pig iron is sufficient as a heat source to heat the molten metal. At lower proportions of pig iron and high proportions of scrap and/or iron ore, it becomes necessary to heat the molten metal externally to compensate for the lack of an internal heat source. There are two ways to do this: one is to supply a carboniferous material, such as coke; the other way is to induce combustion of the carbon monoxide (CO) generated by the carbon-reducing gas-metal reaction, by supplying oxygen (O.sub.2) through an auxiliary nozzle.
Various lances have been proposed and which include an auxiliary nozzle for supplying the oxygen needed for secondary combustion of carbon monoxide. A typical structure of this kind of lance has been disclosed in Japanese Patent First Publication (Tokkai) No. shows 53-102205. The lance disclosed has a plurality of primary nozzles and a plurality of auxiliary nozzles arranged alternatingly. The injecting outlets of the auxiliary nozzles are located higher, i.e. further from the bath surface than the primary nozzles. These primary and auxiliary nozzles adjoin and oxygen passage through the lance. The lance is also provided with a cooling medium circuit for a cooling medium, such as cooling water.
In this known arrangement, the refining operation in the converter is mediated by secondary combustion of carbon monoxide generated in the primary gas-metal reaction. The internal pressure in the converter is held at about atmospheric pressure. On the other hand, the internal pressure in the oxygen passage of the lance is several kg/cm.sup.2 to several tens of kg/cm.sup.2. The primary nozzles are in the form of Laval nozzles. The velocity of the oxygen discharged through the primary nozzle is supersonic. The high discharge velocity of the oxygen ensures that the pressure of the oxygen stream at the molten metal surface will be higher than the static pressure of the slag on the molten metal surface, even though the oxygen is injected from a distance from the molten metal surface of about 1 to 3 m. Specifically, this oxygen jet flows at velocity of over 100 m/sec. Therefore, the oxygen jet churns up the molten metal bath and induces rapid reaction.
On the other hand, the auxiliary nozzles are located higher than the primary nozzles and are essentially straight and untapered. The auxiliary nozzles discharge oxygen at near the speed of sound. Because of their greater distance from the molten metal bath and their straight shape, the auxiliary nozzles produce lower-energy oxygen jets. Thus the oxygen discharged through the auxiliary nozzles can more easily react with the carbon monoxide gas generated by the gas-metal reaction induced by the oxygen jet.
The maximum secondary combustion rate of this conventional blow-refinement lance is about 30% and its heating efficiency is limited to about 20%. However, the effective heating efficiency is significantly lower than 20%. Although this heating efficiency can be improved by adjusting the ratio of pig iron to scrap, the maximum possible increase in heating efficiency is only about 5%.
On the other hand, on the market, the price of scrap is dropping due to continuing increases in supply. Therefore, from the viewpoint of cost, the need for increasing the proportion of scrap is urgent. This requires an improvement in lance design to achieve a higher secondary combustion rate and higher heating efficiency for the molten metal.